Plasma arc torches are widely used in the cutting or marking of metallic materials. A plasma torch generally includes an electrode mounted therein, a nozzle with a central exit orifice mounted within a torch body, electrical connections, passages for cooling and arc control fluids, a swirl ring to control fluid flow patterns in the plasma chamber formed between the electrode and nozzle, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. Gases used in the torch can be non-reactive (e.g. argon or nitrogen), or reactive (e.g. oxygen or air).
In operation, a pilot arc is first generated between the electrode (cathode) and the nozzle (anode). Generation of the pilot arc may be by means of a high frequency, high voltage signal coupled to a DC power supply and the torch or any of a variety of contact starting methods.
One known configuration of an electrode for a plasma arc torch includes an emitting insert (e.g., hafnium) which is press fit into a bore in the electrode. An objective in electrode design is to transfer heat from the hafnium insert and into a cooling medium, which is usually water. Another objective is to control arc root attachment to minimize erosion caused by undesirable arc root attachment to the electrode instead of the hafnium insert.
Electrodes for plasma arc torches are commonly made from copper. Copper is a low cost material that offers good thermal and electrical conductivity. Electrodes for plasma arc torches can also be made from silver. While silver electrodes provide excellent heat transfer characteristics, they tend to be very expensive and not cost effective to use. Copper electrodes are cost effective, but do not have the superior heat transfer characteristics of a silver electrode and thus have a shorter electrode life than silver electrodes.
Several companies manufacture silver and silver/copper composite electrodes using a variety of manufacturing techniques including brazing, soldering, swaging, press fitting and other methods. One company has developed a vacuum brazed copper/silver composite design with a through-hole hafnium insert. Another company has developed a press-fitted silver annulus design with a blind hole hafnium insert. Another company has developed a swaged silver annulus design in a copper holder with copper on the front portion. Another company has developed coined silver electrode design. However, these methods of manufacturing silver/copper electrodes do not produce a sufficiently high-strength joint at the silver/copper interface. In addition, these manufacturing methods result in electrodes that can leak cooling fluid at the silver/copper interface. More significantly, these silver/composite electrodes do not offer the heat transfer characteristics of an all silver electrode.